The present invention relates to a hydraulic brake system with slip control, in particular for motor vehicles, in which at least one master cylinder piston is provided for the operation of a vehicular brake, in which hydraulic medium tapped from the wheel brake during slip control is supplied to the wheel brake from an auxiliary pressure source via a master cylinder gasket acting as check valve, in which the master cylinder piston is acted upon by a servo pressure in the direction of operation proportional to the operating force, and in which inlet bores of the master cylinder communicate with the control pressure side of a control valve.
A brake system with the characteristics referred to above is disclosed in the copending U.S. patent application of J. Belart, J. Burgdorf, D. Kircher, H. W. Bleckmann and L. Weiss, Ser. No. 316,153, filed Oct. 28, 1981, now U.S. Pat. No. 4,416,491 assigned to assignee as the present application. A master cylinder with two master cylinder pistons arranged one behind the other forms a component of this brake system. The master cylinder pistons are operable via a piston rod connected with a brake pedal. If an operating force is exerted on the brake pedal the gaskets of the master cylinder pistons will override so-called expansion bores and, thus, interrupt communication between the working chambers of the master cylinder and an unpressurized reservoir. Upon a further displacement of the master cylinder pistons there will be a pressurization of the working chambers of the master cylinders and the vehicular brakes connected with the working chambers will be applied.
Electromagnetically operable valves are associated with the vehicular brakes. These valves will keep the slip of the wheels down to an advantageous extent in the event of suitable control by slip-monitoring electronics.
Between the pedal-operable piston rod and the master cylinder piston a throttle valve is arranged which essentially comprises a piston which is axially displaceable in the master cylinder bore and which has a pedal-close valve seat and of a valve closure member shaped at the piston rod. As long as there is no danger of a lock-up the pedal force will be transmitted mechanically to the master cylinder pistons via the piston rod and the piston which carries the valve seat of the throttle valve. If the slip-monitoring electronics detect an unduly high slip at one or at several vehicular wheels an electric-motor-driven pressure medium pump will be operated which now will suck hydraulic medium from the unpressurized reservoir, supplying it to the throttle valve. Thus, dependent on the operating force applied to the brake pedal a dynamic pressure will result at the delivery side of the pressure medium pump. This pressure will be transmitted to the pedal-close front sides of the master cylinder pistons and from there the pressure medium may flow via axial bores of the master cylinder pistons and via gaskets acting as a check valve into the brake circuits connected with the working chambers if pressure medium had been tapped from the wheel brakes for the purpose of a pressure reduction.
Moreover the dynamic pressure generated at the delivery section of the pump by means of the throttle valve pressurizes a positioning piston having a larger effective surface than the master cylinder piston and is displaced contrary to the direction of operation against a stop formed in the housing. In doing so, the positioning piston will come into mechanical contact with the piston carrying the valve seat of the throttle valve and will return the latter, if necessary, against the pedal force in the direction of release of the brake. The positioning piston will ensure that there will always remain a sufficient amount of fluid in the working chambers of the tandem master cylinder. Thus, a movement of the master cylinder pistons down to the bottom of the master cylinder will be impossible.
Upon a failure of the pressure medium pump or of the pump drive there will be no pressurization of the positioning piston. Upon an application of the brake in such cases of disturbance, the piston with the valve seat will abut at the positioning piston and displace the same against the force of a relatively weak compression spring in the direction of operation. This displacement will be transmitted to the master cylinder pistons as in normal braking actions. Thus, the working chambers of the master cylinder and the wheel brakes connected thereto will be pressurized. A normal operation of the vehicular brakes will, therefore, be ensured in any case in the above-mentioned conditions of disturbance.
In brake systems of the type referred to above it is known that at first a certain servo pressure must be generated by the brake valve or control valve before pressurizing of the wheel brakes connected to the master brake cylinder.